Bugfix/line label hovers

Rework how mouseover states on line labels work according to [https://www.pivotaltracker.com/s/projects/911874/stories/74780334](this ticket).

The ticket is slightly vague in its requirements, but testing against the grouped timeseries graphs on the following pages, all behave fairly sensibly and predictably.

* http://localhost:3057/performance/vehicle-licensing/service-breakdown
* http://localhost:3057/performance/g-cloud
* http://localhost:3057/performance/register-to-vote/registrations-by-age-group